When in use, the various types of lighting systems for emitting light from the rear are employed as backlights for liquid crystal display devices. Side light devices are especially frequently employed for small and compact liquid crystal display devices in order to save on installation space. For such side light devices, light sources, such as a small fluorescent tubes, are disposed horizontally, and light guide plates are employed that scatter light emitted by the light sources in directions perpendicular to incident directions. FIG. 16 is a diagram showing a conventional side light device used as a backlight for a light transmitting liquid crystal display device. As is shown in FIG. 16, a conventional side light device 70 comprises: a light guide plate 76, to which light is supplied by a light source 72, such as a small fluorescent tube, and from which incident light is reflected toward a display panel 74; and a reflector 78, for effectively employing the light emitted by the light source 72. While the reflector 78 is extended an occupies a position below the light guide plate 76, for the convenience of the explanation this is not shown in FIG. 16.
The light source 72 is accommodated in the reflector 78 when in use, and emits light to the light guide plate 76 through the plane of incidence (not shown). In FIG. 16, specifically for the explanation, the light source 72 is extracted outside the reflector. The end of the light source 72 is accommodated in a lamp socket 80, which assists in retaining the light source 72 and holding it inside the reflector 78.
The display panel 74 includes a diffusion sheet 74a, a prism sheet 74b and a liquid crystal display panel 74c. The light guide plate 76 is generally designed to print a white dot pattern, and the light produced thereby is reflected and scattered, substantially perpendicularly, so that it irradiates the display panel 74c as uniformly as possible. A lead line 82 from the end of the lamp socket 80 is introduced to enable, from the outside, the supply of power.
FIG. 17 is a diagram showing the detailed configuration of a small fluorescent tube used as the light source 72. As is shown in FIG. 17, the light source 72 is formed of an almost hollow glass tube, and includes a luminous portion 72a, which is coated with a fluorescent material containing a rare earth that emits a white light; and a graphitized portion 72b, which is formed at the end of the light source 72, adjacent to the luminous portion 72a. Since the light emission by the fluorescent material is not satisfactorily transmitted through the graphitized portion 72b, the light emitted from the light source 72 becomes non-uniform. The lead line 82 extended from the end of the light source 72 to supply power to the light source 72 is connected to a soldered portion 72c. In the light source 72, the graphitized portion 72b and the soldered portion 72c constitute a non-luminous portion 72d. 
FIG. 18 is a cross sectional view, taken along the arrow line A in FIG. 16, of the conventional side light device that employs the above structured light source 72. As is shown in FIG. 18, the light source 72 is supported by the lamp socket 80, and radiates light to the light guide plate 76. A non-displaying area 84 is formed on the outer wall of the light guide plate 76 in order to prevent the non-uniform emission of light from the outer circumference. The inside of the non-displaying area 84 serves as a displaying area 86 for a liquid crystal display panel.
The lamp socket 80 shown in FIGS. 16 to 18 is formed of various types of materials that comply with a flame-retardant standard, such as V-0, V-1 or V-2. In many cases, in order to comply with the above flame-retardant standard, a flame-retardant, such as antimony oxide, phosphoric ester, a nitrogen-content compound, a halogen compound, a polyol compound or zinc borate (ZnO•2B2O3•3.5H2O), and a filler, such as mica, talc, silica or alumina, are mixed together and used, or either one is filled. Generally, a lamp socket material and a flame-retardant are added to obtain a required flame-retardant property. Thus, the transmittance of light is lost, and in correlation with the above described non-luminous portion, satisfactory light irradiation is not performed at the corners of the light guide plate 76, i.e., corner shading 88 occurs.
FIG. 19 is a detailed diagram showing the corner shading 88 of one of the corners of the light guide plate 76. Recently, an improvement in display quality and a reduction in the thicknesses of devices and the widths of frames have been considered for liquid crystal display devices, particularly for light transmitting liquid crystal display devices. Therefore, the non-luminous portion 72d must be within the part of the light guide plate 76, and as a result, the lamp socket 80 is extended inside the displaying area (a≧b). In this case, “a” denotes the length of the lamp socket 80 and “b” denotes the distance from a frame 92 to the end of a displaying area 86.
Because of the designs of the light source 72 and the peripheral members, the non-luminance portion 72d, near the electrode of the light source 72, must also be extended to the displaying area 86 of the light guide plate 76, and as a result, greater shading occurs in the vicinity of the light guide plate 76 (a+c≧b). In this case, “c” denotes the length of the non-luminous portion 72d that extends out and over the lamp socket 80. And when the non-luminous portion 72d is extended out to the displaying area 86, corner shading 88 occurs.
In order to provide a thin display device, such as a thin, light transmitting liquid crystal display device having a narrower frame, corner shading 88 would greatly affect the uniformity in the luminance of a display, and the display portions, particularly the corners adjacent to the light source 72, would become dark. Therefore, corner shading 88 is regarded as a barrier to the downsizing of a liquid crystal display device and the increasing of its screen size.
The reason corner shading 88 is generated will be further explained by referring to FIG. 19. Before entering the light guide plate 76, the diffused light emitted by the light source 72 is refracted, as follows, in accordance with Snell's law.
[Equation 1]n1 sin θ1=n2 sin θ2  (1)wherein ni denotes the refractive index of medium i and θi denotes the refractive angle at the medium i. When n1=1.0 (air) and n2=1.49 (acrylic resin), θ2=42.15° is obtained. Therefore, when an opaque lamp socket 80 is employed for the light guide plate 76, corner shading 88 occurs at a point about 48° from the end of the effective non-luminous portion, including the lamp socket 80. Further, in this invention, since in order to prevent corner shading 88 it is required that shading for the lamp socket 80, at least in the displaying area 86, should not be formed, in FIG. 19 the shading must be retracted down and into a d(1+tan 42°) shaded area 90.
Therefore, the use of a transparent lamp socket has also been considered. However, so long as the non-luminous portion 72d of the light source 72 extends out into the displaying area 86, corner shading 88 can not be reduced, regardless of whether or not a transparent lamp socket is employed. That is, since in order to effectively reduce corner shading 88 the shading produced by the non-luminous portion 72d must be effectively removed, not only must a transparent lamp socket be employed, but it is inevitable that light from the light source must be transferred to the portion of the non-luminous portion 72d wherein the shading is formed.
Therefore, a demand exists for a lighting system that can effectively reduce corner shading 88, a liquid crystal display device that includes such a lighting system, and a lamp socket therefor.